1. Technical Field of the Invention
The present invention relates generally to a position shift control apparatus designed to shift the position of an object joined to an output shaft connected to an electric motor through a torque transmission mechanism such as a speed reducer.
2. Background Art
In recent years, there has been an increasing tendency in automotive vehicles to replace mechanical drive systems with electrical ones equipped with an electrical motor for meeting requirements of space saving-designs or improvement of assemblability and controllability thereof. For example, Japanese Patent First Publication No. 2004-23890 (US 2006/0033464 A1) discloses an example of the electrical drive system designed to use an electrical motor to drive a range shift mechanism for automotive automatic transmissions. This system works to drive the range shift mechanism through an output shaft joined to an output shaft of the motor through a speed reducer to shift the gear range of the automatic transmission. The motor is equipped with an encoder to measure an angular position of the motor. When it is required to change the gear range of the automatic transmission, the system monitors a count of pulse signals outputted from the encoder and rotates the motor until a target angular position corresponding to a selected one of the gear ranges is reached.
The amount of rotation (i.e., a rotated angle) of the motor is converted by a torque transmission mechanism made of the speed reducer, etc. into that of the output shaft (i.e., a manipulated variable in the range shift mechanism). The torque transmission mechanism usually has mechanical backlash or play. For instance, the speed reducer has the backlash between gears installed therein. In the case where an end of the rotor of the motor which is of a noncircular cross section is fitted within a hole formed in the end of the output shaft to establish a mechanical joint therebetween, a small amount of clearance is typically formed between the end of the rotor and the hole of the output shaft for facilitating ease of insertion of the end of the rotor into the hole. The mechanical backlash or play in the torque transmission mechanism will result in an inevitable error in controlling the angular position of the output shaft (i.e., the manipulated variable in the range shift mechanism) based on the count value of the encoder, thus leading to a decrease in accuracy of operating the range shift mechanism.
In order to avoid the above problem, Japanese Patent First Publication No. 2004-308752 (US2006/0207373 A1) teaches a motor control system which perform an initial motor drive operation which rotates the motor until a limit of a rotatable range, as defined by a detent mechanism, is reached after start-up of the system and learns an angular position of the motor upon reach of the limit as a reference angular position of the motor, thereby compensating for the error arising from the backlash and play in the torque transmission mechanism.
Japanese Patent First Publication No. 2002-310294 teaches use of a neutral start switch (NSW) which measures each of gear shift positions in the range shift mechanism which correspond to the gear ranges of the automatic transmission, respectively, to learn the correspondence between the count value of the encoder and each of the gear shift positions, as measured by the NSW.
The motor control system, as taught in Publication No. 2004-308752, requires the execution of the initial motor drive operation every start-up of the system, thus resulting in an increase in mechanical load on the detent mechanism or parts of the torque transmission mechanism, which leads to a greater concern about the durability thereof.
The system, as taught in Publication No. 2002-310294, is designed to use an average of count values of the encoder, as sampled upon changes in on-of state of the NSW during a normal and a reverse rotation of the motor, respectively, to learn the reference angular position of the motor. The average indicates the center of each of the gear shift positions. The system, however, does not compensate for an error in learning the reference angular position at all which arises from the angle at which the NSW is mounted on the automatic transmission. Further, when the NWS is changed from the off-state to the on-state, torque which is created by elastic pressure, as produced by the detent mechanism, and acts on the output shaft of the motor is identical in orientation with that outputted by the motor, thus resulting in a difficulty in knowing whether the amount of backlash or play in the torque transmission mechanism is increasing or decreasing during the rotation of the motor. This also results in an additional error in learning the reference angular position of the motor.